ABSTRACT The shared epitope (SE) is the single most significant risk factor for severe rheumatoid arthritis (RA), yet its mechanism of action is not completely understood. During budget years 01-05 of this R01 project, we identified the SE as a signal transduction ligand that specifically interacts with a well-defined binding site on cell surface calreticulin (CRT) and activates intracellular signaling events that lead to Th17 polarization and osteoclast activation. When administered to mice with collagen-induced arthritis, a synthetic SE ligand enhanced the severity of erosive joint destruction. Furthermore, based on structure-function characteristics of SE-CRT interaction, we have therapeutically targeted the pathway with CRT-binding small molecules that potently inhibited the arthritogenic pathway with resultant amelioration of experimental arthritis in mice. Based on preliminary results obtained in the current funding cycle of this project, in years 06 ? 10 we will carry out experiments that will advance our understanding of the molecular mechanisms and functional outcomes of the SE-activated pathway. Additionally, we will elucidate the signaling and functional effects of a protective epitope (PE) ligand that we recently discovered. This ligand appears to counteract the SE pathway and may explain a long-established but poorly understood epidemiologic association between particular HLA-DRB1 alleles and protection against RA. The hypothesis of this project states that in RA: A. Disease-facilitating and disease-protective HLA- DRB1alleles code for distinct ligands, which transduce reciprocal cell activation and differentiation signals; B. The SE pathway activates protein citrullination. The research plan in the coming 5 years will address this hypothesis with the following specific aims: 1. To delineate the respective signaling pathways activated by the SE and PE ligands and determine their functional effects on cell differentiation; 2. To analyze signature HLA- DRB1-coded ligand-activated transcriptomes using an RNA-seq approach; 3. To characterize the mechanism of a recently discovered SE-activated protein citrullination pathway, and determine its in vivo effects. Successful completion of the proposed research will illuminate fundamental questions concerning HLA-disease association, provide important new insights into the pathogenesis and etiology of RA, and set the stage for designing novel therapeutic strategies.